PROJECT SUMMARY/ABSTRACT The broad, long-term objective of the proposed research is to identify somatic genetic mutations that occur in brain regions of individuals with autism spectrum disorder (ASD), schizophrenia (SZ), and bipolar disorder (BD). Ultimately the discovery and characterization of somatic mutations may help us to understand the etiology of these disorders and eventually lead to improved therapeutic strategies and diagnostic markers. ASD, SZ, and BD are all commonly occurring neuropsychiatric disorders that have a large genetic basis (with estimated heritability of ~75% to 80% for each condition). However, relatively few DNA variants have been identified that have causal roles in the etiology. We hypothesize that somatic mosaicism--the occurrence of mutations in selected body regions after conception--occurs in brain and contributes to the etiology of ASD, SZ, and BD. Specific Aim 1 is to identify the nature and extent of somatic mosaic mutations across brain and body regions in postmortem samples from apparently normal individuals. We will assess four categories of somatic variation: (1) single nucleotide variants (SNVs), (2) structural variants (SVs) including copy number variants, (3) L1 retrotransposition events, and (4) mitochondrial heteroplasmy. These types of variation will be detected using whole genome sequencing (Years 1 and 2), single molecule imaging with DNA nanochannels (Year 2), and single nucleotide polymorphism (SNP) arrays. Samples include brain regions (e.g. prefrontal cortex and cerebellum) and organs (e.g. heart and kidney). After identifying somatic variants we will perform rigorous validation. Specific Aim 2 is to identify the nature and extent of somatic mosaicism in genomic DNA from individuals with ASD, SZ, and BD. The same approaches for discovery and validation will be applied as in Aim 1. Specific Aim 3 is to functionally categorize somatic variants, particularly those that are predicted to disrupt the functions of genes previously implicated in those disorders. One approach is single-cell RNA-seq (to determine the consequence of the mutation on transcription, and to infer the cell type of origin of the somatic variant). Another approach uses neurons (or glia) derived from induced pluripotent stem cells (iPSCs) and stably expressing the wildtype or mutant forms of the somatic variants. These studies will help to establish the role of somatic mutation in neuropsychiatric disorders, including the functional consequences of such variation.